Flash memories, both embedded and standalone, have been growing in importance in modern day electronics, with most of the development focusing on re-channel flash memories. Focus has turned to p-channel flash memories, as illustrated in FIG. 1, for high speed and low power operation. Adverting to FIG. 1, the p-channel flash memory includes p+ source/drain regions 101 in a substrate 103, a gate 105 with spacers 107, and a charge storage stack 109 which may be formed of various storage materials including oxide-polysilicon-oxide and oxide-nitride-oxide. Programming of the cell is accomplished by channel hot-hole induced hot electron injection (CHE). Hot-holes cause electron-hole pair generation by impact ionization at the drain side. The generated holes are drifted to the drain whereby the electrons are accelerated in the channel and become hot enough to overcome the barrier of the tunneling oxide. The p-channel cell is preferred over the n-channel cell due to a greater vertical electric field across the tunneling oxide, which favors electron injection, which in turn increases the probability/number of elections being injected. The cell is then erased by Fowler-Nordheim (FN) tunneling.
To increase electron injection efficiency, programming utilizing band-to-band tunneling induced hot electron (BBHE) injection has been employed. Improved injection efficiency arises from the higher vertical electric field at the electron injection point. Table 1 illustrates an example of various operating conditions for programming using BBHE.
TABLE 1Vcg (V)Vd (V)Vs (V)Vsub (V)Program (CHE)4.504.54.5Program (BBHE)6−3float1.5Erase (FN)−5.5666Read1.501.51.5FIGS. 2A through 2C illustrate the energy band diagrams for the device of FIG. 1 extracted along A-A′, B-B′, and C-C′, respectively, during programming using BBHE. As illustrated in FIG. 2A, band-to-band tunneling (BTBT) of electrons occurs from the drain to the surface. The electrons are then accelerated to the source, as illustrated in FIG. 2B. Some electrons, however, will gain enough energy to overcome the tunnel oxide barrier, as shown in FIG. 2C.
Efforts to improve BBHE have included increasing the gate-to-drain overlap region/area or using narrower bandgap materials at the drain side to increase the number of BTBT electrons. However, these approaches result in earlier punch-through and/or increasing junction leakage, which in turn limits scaling or results in larger memory cells. Additionally, forming narrow bandgap materials in the drain and extending it sufficiently below the gate for adequate gate-to-drain overlap is difficult.
A need therefore exists for p-channel memory cells exhibiting improved BBHE for programming while maintaining a compact cell size and good device characteristics, and for enabling methodology.